1. Field of the Invention
The invention comprises an apparatus attached to the back of a vehicle for reducing the fluid drag thereon by creating at least two vortices which turn the flow inwardly after it passes the sides of the body.
2. Description of the Prior Art
A number of efforts have been made to reduce the aerodynamic drag on blunt bodies such as semi-trailers and trailer vans. In general vans have a blunt rear end in order to improve ease of fabrication of rear loading doors and also so that the trailer can butt directly against loading docks. Blunt ended moving bodies have high drag as a consequence of relatively lower pressures acting on the rear base of the vehicle. The base pressure results in base drag and the reduction of this drag has received some attention by previous investigators and inventors. Boat tailing, which involves adding a tapered extension backward from the blunt base, has long been known to reduce base drag. However, those devices are bulky and must be removed to permit loading of the truck. U.S. Pat. No. 4,257,641 entitled VEHICLE DRAG REDUCER discloses a boat tail which replaces the conventional system of doors. Similarly, U.S. Pat. No. 4,451,074 describes a collapsible boat tailing airfoil in which the airfoils can be folded away from the base and against the side of the trailer when the vehicle is backed into a loading dock. Unfortunately boat tails are relatively large structures and require the substantial deployment of hardware. As a compromise the boat tail may be reduced in size by not tapering the end to a point such as described in U.S. Pat. No. 4,214,787 entitled DRAG REDUCING APPARATUS. Such a device could be referred to as a partial boat tail. Alternatively, Freuhauf produced an experimental FEV 2000 tractor semi-van trailer vehicle which was built in 1981. An interesting description of the FEV 2000 vehicle is found in an article entitled "The Truck of the Future" which appeared in the June 1985 edition of High Technology Magazine on pages 28 through 33. However, base drag reduction using partial boat tailing is not as great as that achieved when using full boat tailing. This performance degradation may be somewhat offset by using a vortex generator on the partial boat tail such as described in U.S. Pat. Nos. 3,741,285 and 3,776,363. Other devices described in the patent literature but not believed to be as relevant to the present invention are described in U.S. Pat. Nos. 2,514,695; 3,934,923; 3,960,402; 3,971,586; 3,999,797; 4,113,299; 4,316,360 and 4,451,074.
Another approach to reducing base drag is to bleed air into the base area. Studies undertaken by Sykes, in the 1969 Proceedings of the First Symposium on Road Vehicle Aerodynamics at the City University in London, have shown that significant reductions in base drag can be achieved. Unfortunately, when estimates are made of the power required to supply the air to the base region, the concept becomes impractical and uneconomical.
Another alternative method to reduce base drag was proposed by W. T. Mason, Jr. and P. S. Beebee, in the Proceedings of the Symposium on Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, sponsored by General Motors Research Laboratories on Sept. 27-28, 1976. The concept was to install a single splitter plate either vertically or horizontally extending perpendicular off the rear base of the truck. The purpose of the plate was to allow the flow separating from the side of the truck to reattach to the vertical plate or the flow separating from the roof and the bottom of the truck to reattach to the horizontal plate. Unfortunately, very little reduction in base drag was achieved with that concept.
Four of the embodiments studied by Mason and Beebee are illustrated in FIGS. 1A through 1D and labeled as "Prior Art." FIG. 1A illustrates a single vertical splitter. Unfortunately, to be effective a single vertical splitter has to extend an unreasonably long distance behind a van. Moreover, the inherent instability of the vortices created on both sides of the single splitter causes the resultant drag reduction to be relatively poor. FIG. 1B is similar to FIG. 1A except that the single splitter is located in the horizontal plane. This technique was also believed to be ineffective due to the extreme length of the single splitter and the turbulence generated beneath the truck carriage. FIG. 1C illustrates another prior art technique in which turning vanes are used to turn the airflow around the sides inwardly thereby reducing the effective base drag area of the van. FIG. 1D illustrates another prior art technique examined by Maso and Beebee in which a dead air space cavity was effectively formed behind the end of a semi-trailer van by extending the plane on the side and top surfaces of the vehicle. In general the results obtained by the prior art techniques 1A through 1D were disappointing, especially when viewed in light of the results obtained by the present invention.
The splitter plate concept has been shown to be effective in reducing the base drag of two-dimensional bodies such as the blunt trailing edge of an airplane wing. See for example, M. Tanner, Progress in Aerospace Science, Vol. 16, No. 4, pp. 369-384, Pergamon Press, 1975. Apparently the flow behind a truck is more complicated than a flow behind a simple two-dimensional body. Another approach using plates to reduce base drag is to attach plates so that they are flush to either the sides or the roof or the floor of a vehicle and then extended backwards. In an experiment reported by D. J. Maull, in the Proceedings of the Symposium on Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, sponsored by General Motors Reseach Laboratories in 1978, such plates provided relatively modest but not sufficient reduction in base drag to be economically viable.
An article entitled The Effect of a Rear-Mounted Disc on the Drag of a Blunt-Based Body of Revolution, by W. A. Mair, and published in The Aeronautical Quarterly, Nov. 1965 has shown that a disc of revolution mounted on a base of revolution can reduce base drag substantially. Such a device is shown in FIGS. 1E, 1F and 1G and is labeled "Prior Art". The reduction in drag is believed to be the result of a vortex ring eddy trapped between the base of the body and the disc as shown in FIG. 1G. The vortex ring is believed to help turn the flow inward downstream of the base and, hence, reduce the effective base area of the body of revolution. Unfortunately, a simple rectangular plate cannot be attached to the base of a truck with similar success, since the axis of a vortex ring cannot be bent to a rectangular configuration to conform to the space between the base and the rectangular plate. The Mair concept does, however, describe the use of trapped vortices to reduce the base drag of vehicles.
The disappointing results of the prior art suggest that no one, to date, has successfully trapped a vortex behind a body similar to a semi-trailer or trailer van. The present invention appears to have been successful in efficiently trapping a vortices and obtaining drag reduction in the neighborhood of 10.2%. This has resulted in substantial drag reduction all of which could have a significant impact on the cost of trucking and our quality of life. In the United States semi-trailer vans expend over 10 billion gallons of diesel fuel per year. A needless waste of fuel increases dependency on imported oil, as well as adding needless diesel exhaust carcinogens into the atmosphere. It had previously been difficult to reduce the drag on blunt bodies by trailing devices given federal and state regulations. However, recently the United States Congress has enacted legislation which permits trailer/semi-trailer length limits to be exceeded if the purpose is to improve fuel efficiency. It appears that the present invention can reduce the aerodynamic drag of a semi-trailer or trailer van in excess of 10%. This could result in a nationwide yearly savings of diesel fuel in excess of 1 billion gallons.